Many different tunnel structures have been developed, including tunnel diode structures. A typical tunnel diode is formed from a p-n junction in which both n and p sides are degenerate (i.e., very heavily doped with impurities). The high doping levels bend the conduction and valence bands over a short distance (e.g., on the order of 10 nm, or less) such that conduction band electron states at the Fermi level on the n side are aligned with valence band states at the Fermi level on the p side. This phenomenon allows charge carriers to tunnel across the depletion layer upon application of an arbitrarily small forward or reverse bias across the p-n junction. Beyond a particular forward bias level, the available charge carrier tunnel states do not overlap, resulting in a region of negative differential resistance (i.e., where the current exhibits a decrease in value when the value of the voltage is increased).
In general, several conditions must be satisfied to achieve such a tunnel diode structure. For example, the n and p sides of the tunnel structure must be degenerately doped. This condition requires a combination of heavy doping levels and low dopant ionization energies. In addition, the tunnel distance must be sufficiently short to enable charge carriers to tunnel across the depletion region with a relatively high probability. This condition requires very heavy doping levels. One or more of these tunneling conditions cannot be satisfied in certain semiconductor material systems. For example, in many semiconductors (especially wide bandgap semiconductors, such as III-V nitride and II-VI semiconductors) heavy doping of the n side or the p side, or both, is difficult or impossible to achieve. In addition, in these semiconductor material systems, dopant ionization energies often are relatively high, especially for p-type dopants. The combined effect of these limitations makes it difficult or impossible to fabricate tunnel diode structures that operate at moderate reverse voltages, much less operate at nearly zero voltage.